Try to start a new project and import the old project into the new one to check if this message appears.
Still the same problem. I realized though that I get another message when opening the project which says:
After Effects warning: effect control conversion required in effect "u". Some effect controls will be reset. To avoid silently resetting these same effect controls on all subsequently loaded projects, save this project and then quite & re-launch After Effects. ( 25 :: 0 )
So I started uninstalling all of my plugins one by one and it seems like Red Giant Universe is the problem. They just released a new version so that might be what caused it. I'll open a support ticket with them as well.
I solved it by removing all of the old Red Giant Universe effects, and then adding them again
Thanks fro posting. Just having the same issue this morning